1. Field of the Invention
The present invention relates generally to the non-destructive evaluation of surface contamination and corrosion levels and, more particularly, to an apparatus and method for monitoring optically stimulated electron emission.
2. Description of the Related Art
Surface quality inspections are presently achieved by one of a variety of methods and techniques. One method is to omit any type of verification and just rely on tests of samples of the bonded/coated product to determine the integrity of the bond. In many cases, visual inspection for foreign particles or substances is the method of choice. In some cases, surfaces that are to be bonded are subjected to inspection by “black light” for fluorescence caused by contamination. The fluorescence occurs at visible or invisible wavelengths. A variation of the black light inspection uses fluorescence which is caused by a pulsed ultraviolet laser. Other prior art methods, such as ellipsometry, utilize changes in the direct reflected light. Another method to detect surface contaminants requires monitoring changes in infrared emissivity. These changes are sometimes emphasized by using grazing-angle infrared radiation to illuminate the sample. Methods and techniques based on reflection-infrared spectrometry are used to evaluate the chemical compounds causing the contamination.
Ionic contamination is sometimes evaluated by solvent extraction of the material from the surface of sampled product and measurement of the conductivity change of the solvent. Other contaminants are detected following solvent extraction by gravimetric measurements of non-volatile residues that remain after evaporation of the solvent in which the sample was previously immersed or washed. In another similar method, the washing solvent is passed through a filter, and the gain in weight of the filter after drying constitutes the measurement of contamination. Another method, known as the “water break test,” uses the contact angle or the regularity of the edge of the solvent film as it drains from the sample under test. Another method, known as optically stimulated electron emission (OSEE), is currently being used to detect contamination. This method is described in U.S. Pat. Nos. 4,590,376 and 5,393,980. OSEE is also described in the paper by Perey, D. F., entitled “A Portable Surface Contamination Monitor Based On The Principle of Optically Stimulated Electron Emission (OSEE),” JANNAF Propulsion and Joint Subcommittee Meeting 1996, p. 8, December 1996.
If the samples of a production run are small enough, such samples can be placed in a high-vacuum system that permits a variety of methods to be used to detect and, in some cases, chemically analyze contaminants.
Another well-known technique is the use of Kelvin Probes to detect variations in the work function of samples of a production run.
Many of the previous known prior art methods are used only on production samples because these methods actually compromise (i.e. alter or damage) the sample. Thus, these methods are not used to inspect all of the production run.
Some known prior art methods, notably the solvent extraction method, require laboratory processing following the measurement process thereby increasing the processing time.
Ellipsometry is another method currently used in industry. Ellipsometry provides the best results when applied to relatively flat, smooth surfaces. However, ellipsometry does not yield highly accurate results when applied to surfaces that are not relatively flat and smooth.
Infrared methods generally have low sensitivity to small amounts of contamination but perform better when relatively high amounts of contamination are present.
The visual inspection methods, including black light inspection, can be difficult to quantify and may yield inconsistent results.
Some of these known prior art methods and techniques require fluorescent tracer materials to be added to process chemicals in order to provide the desired results.
Furthermore, only infrared reflection spectrometry and the high vacuum methods provide more than a single indicator of quality in a measurement.
Thus, it is an object of the present invention to provide an apparatus and method for performing quality inspections on a test surface that addresses the deficiencies of the known art method and apparatuses.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.